Modern mobile phones are getting smaller and smaller and thus the interaction between antenna, phone body and user will become more important than earlier. It is well known that the size of an antenna is critical for its performance. There is also normally a requirement today that two or more frequency bands are supported.
Mobile phones generally exchange radio signals with a radio base station. Some signal exchange occurs during standby when no call is going on and the phone is located for instance in the hand, in a pocket, or at the waist of the user. Signal exchange of course occurs when a call is going on and the phone is then typically located between the ear and mouth of the user, or still in a pocket or at the waist of the user with an earpiece and a microphone connected.
A fundamental and efficient antenna type for mobile telephone is the monopole consisting of an antenna whip having a length generally a fraction of a wavelength and a phone circuit board acting as a corresponding ground conductor. Among them a length of half a wavelength was used in many older phones and gives a very low feeding current (corresponding to high impedance) with low currents on the telephone body or circuit board. This type of antenna provides very low electromagnetic fields on the phone itself and thus little interaction with head, hands etc close to the phone. However, the size is much bigger than complying with modern telephone design so generally much smaller antennas are required for the sake of easy handling.
However, since the small antenna has to radiate the same power as a large one (due to the requirements of the phone system) the currents or voltages (depending on the type of antenna) on the small antenna will be larger. This is especially true when the structure is small as compared to a wavelength. Thus the possible interaction with various objects close to the antenna will inherently be larger and so will the currents along the phone body or circuit board. This applies to all typical screeners in telephone surroundings, which means that the electromagnetic fields of the antenna will interact significantly with the user's body during call mode. The interaction would generally occur during standby as well as if the phone is close to the user's body.
When dealing with interaction between the telephone antenna and its immediate surroundings, the electromagnetic near field of the antenna is more important than the far field. In this interaction, there are at least two different quantities to consider.
One is the power loss in the surroundings consisting of losses in for instance a table, a bag, or in a hand, a head and other human tissue. Such losses have to be considered when designing an antenna for a mobile system, as the phone systems require a certain power level (such as 2W peak and 0.25 W in average for GSM). Another quantity is Specific Absorption Rate (SAR) which is relevant in countries where there is legislation and regulation defining SAR upper limits as the power loss per a certain unit of body tissue, generally quantified as an average in watts per a certain amount of body tissue. For instance, the FCC (Federal Communications Commission) in the USA requires that SAR be less than 1.6 mW in average per gram of body tissue. Different antennas and phones exhibit different SAR for the same radiated power. According to standards (FCC, CENELEC and others), SAR is measured inside a dummy head.
Due to the general desire of obtaining as large signal strength as possible of a mobile phone, an internal antenna element is traditionally designed to be as large as possible, i.e. extending beyond the PCB or at least right up to an edge of the PCB.